Indirect with storage tank issues

AirborneTrav

So I have run into an interesting problem that I was hoping someone could shed some light on. My company services a bed and breakfast that was running into a problem with one regular guest: he would show up, run the gigantic tub in one of the rooms until the 80 gallon Weil McLain indirect was cold, complain that he didn't have enough hot water, and get a refund. He was pretty much the only one that complained, but they ended up having us add an electric water heater as a storage tank, that we heated with a stainless 007 connected to the bottom inlet, circulating hot water from the indirect until the thermostat was satisfied. This has worked great since last June or July. He has stayed multiple times and hasn't been able to demand a refund. We got a call from them last week that another guest complained that they were only getting lukewarm water in the shower. The maintenance guy on site checked it out
, got hot water at the shower, and chalked it up to the guy BSing him, until it happened with another guest. We initially suspected the mixing valve- replaced it, and it worked fine, until it didn't. I came here today and realized that while the stainless 007 is NOT running, we get tepid water out of the mixing valve. The second it kicks on, the mixed pipe out of the valve gets right up to temperature. I have posted a diagram of our setup, and it's open to critique. I really don't untestable how this just started happening, nor why it is happening in the first place. Anyone have any ideas?